The availability of high performance optical amplifiers such as Raman amplifiers and the erbium-doped fiber amplifier (EDFA) has renewed interest in the use of wavelength division multiplexing (WDM) for optical transmission systems. In a WDM transmission system, two or more optical data carrying channels, each defined by a different carrier wavelength, are combined onto a common path for transmission to a remote receiver. The carrier wavelengths are sufficiently separated so that they do not overlap in the frequency domain. The multiplexed channels are demultiplexed at the receiver in the electrical or optical domain. Demultiplexing in the optical domain requires using frequency-selective components such as optical gratings or bandpass filters. Typically, in a long-haul optical fiber system, the set of wavelength channels would be amplified simultaneously in an optical amplifier based repeater.
One class of optical amplifiers are rare-earth doped optical amplifiers, which use rare-earth ions as a gain medium. The ions are doped in the fiber core and pumped optically to provide gain. The silica fiber core serves as the host medium for the ions. While many different rare-earth ions such as neodymium, praseodymium, ytterbium etc. can be used to provide gain in different portions of the spectrum, erbium-doped fiber amplifiers (EDFAs) have proven to be particularly attractive because they are operable in the spectral region where optical loss in the fiber is minimized. Also, the erbium-doped fiber amplifier is particularly useful because of its ability to amplify multiple wavelength channels without crosstalk penalty.
The gain characteristics of a rare-earth doped optical amplifier depend on the dopants and co-dopants used to make the fiber core, the particular rare-earth ion employed, and the pumping mechanism that is used. These parameters can be controlled to produce optical amplifiers having different gain profiles. FIG. 1 shows the gain of a conventional EDFA as a function of wavelength over a spectral region of about 1525 nm to 1580 nm. This spectral region is one in which the carrier wavelengths are often located. Clearly, the gain undergoes substantial variations over the spectral region. These variations are exacerbated when many different channels are used which extend over a wide bandwidth.
Unequal gain distribution adversely effects the quality of the multiplexed optical signal, particularly in long-haul systems. For example, insufficient gain leads to large signal-to-noise ratio degradations while too much gain can cause nonlinearity induced penalties. Gain equalizers are therefore used in optical amplifier designs to ensure constant gain over the usable wavelength range. When a bandwidth substantially exceeding 45 nm is required, gain equalizers alone cannot provide the desired performance.
One approach for obtaining broadband constant gain involves dividing the multiplexed signal into two or more bands that occupy a narrow portion of the total bandwidth occupied by the original multiplexed signal. Each band is then amplified individually with its own dedicated optical amplifier, which is tailored to provide a relatively flat gain across its respective portion of the bandwidth. Once amplified, the bands are recombined so that the resulting amplified multiplexed signal can continue traveling along the transmission path. One example of this technique employs two EDFAs that provide reasonably uniform gain over wavelengths between 1525-1565 nm, and 1565-1618 nm, respectively.
One problem that arises when the multiplexed signal is divided into different bands is that distribution and insertion losses cause an increase in noise. This loss can increase as the number of bands into which the signal is divided increases. For example, in double-band amplifiers a combination of an optical circulator and a reflecting filter is typically used as the band splitting device. These devices introduce a .about.2 dB insertion loss on the low wavelength band and 1 dB insertion loss on the high band.
The effects of insertion and distribution losses may be somewhat mitigated by using a single amplifier to amplify the original multiplexed signal prior to its division into separate bands. That is, a preamplifier may be used to compensate for the losses. However, this is not an entirely satisfactory solution because the preamplifier, of course, will provide optimal gain for only a portion of the bandwidth of the multiplexed signal. As a result, the noise figure for the preamplifier and amplifier combination is still highly effected by losses due to bandsplitting.